Corrosion protective cleaning agent for tin-plated steel

ABSTRACT

The invention concerns an aqueous corrosion-protective cleaning solution for tin-plated steel, in particular for tin-plated steel cans, the solution containing complex fluorides of the elements boron, titanium, zirconium, and hafnium; non-ionic surfactants; and corrosion inhibitors and having a pH within the range from 3 to 6. The invention also concerns an aqueous concentrate for preparing the solution by dilution with water and a method of cleaning tin-plated cans using the solution.

FIELD OF THE INVENTION

This invention relates generally to the cleaning and corrosion-proofingof tin-plated steel, more particularly food or beverage cans of thismaterial, so-called "tin cans", between the forming and lacquering stepsof the can manufacturing process.

TECHNICAL BACKGROUND AND RELATED ART

Tin cans are normally produced by preliminary forming, deep drawing andsmoothing. They have a desirable bright surface so that, after coatingwith a clear or opaque organic lacquer or printing of the outer surface,they are suitable as an attractive pack. The sequence of process stepsinvolved in the production of tin cans normally comprises offwinding thestrip of tin plate provided with a layer of protective oil from thecoil, applying drawing lubricants, preliminary forming into a cup anddeep drawing and smoothing to the final shape. In the deep drawing andsmoothing steps, cooling lubricants, such as water or aqueous emulsions,are normally also used to facilitate the deep drawing process. Afterforming, residues of the protective oil and deep drawing lubricants andalso metal dust are removed in a cleaning steep. After the cleaningstep, the containers pass through one or more water rinsing stages andare then dried in a drying oven. This is followed by lacquering in oneor more stages and by decorative printing of the outer surface.Accordingly, the metal surface has to be of such quality that thelacquer has sufficient adhesion and affords reliable protection againstcorrosion.

In the production of such cans, however, it has been found that rustspots can develop during or before drying, particularly if the cans arecleaned with an acidic medium (pH 3-5), too much water is retained incertain areas or if, during stoppage of the production line, theindividual process steps are not completed sufficiently quickly. Rustspots can thus develop. They are visible through the lacquer and promotepoor lacquer adhesion, so that the product packed in such a containersoon becomes unfit for consumption.

According to the teaching of EP-B-161 667, this problem can be solved innon-tinned steel cans, so-called black plate cans, by treating the cansafter the actual cleaning step with an aqueous corrosion-inhibitingsolution which contains 10 to 5,000 ppm of aluminum ions, 10 to 200 ppmof fluoride ions and up to 1,000 ppm of ions of at least one of themetals titanium, zirconium and/or hafnium and which has a pH value of 2to 5.5.

The problem addressed by the present invention was to provide atreatment solution, a concentrate for its preparation and a treatmentprocess with which tin cans could be cleaned and provided with a surfaceprotected against corrosion in a single treatment step so that corrosionof the surface before lacquering would be prevented and firm lacqueradhesion would be promoted.

BRIEF SUMMARY OF THE INVENTION

In a first embodiment, the present invention relates to an aqueoustreatment solution for tin-plated steel which contains:

100 to 400 ppm of complex fluorides of the elements boron, titanium,zirconium and/or hafnium;

100 to 2,000 ppm of nonionic surfactants;

100 to 1,000 ppm of corrosion inhibitor;

and, as the balance, water or an aqueous solution of other activeingredients or auxiliaries and which has a pH value in the range from 3to 6.

DETAILED DESCRIPTION OF THE INVENTION

The following concentration ranges are particularly preferred for theindividual active ingredients:

150 to 300 ppm of complex fluorides of the elements boron, titanium,zirconium and/or hafnium;

300 to 1,000 ppm of nonionic surfactants;

150 to 500 ppm of corrosion inhibitor.

The addition of aluminum ions to the cleaning solution in aconcentration of about 50 to 300 ppm and, preferably, about 80 to about200 ppm has been found to have positive effects.

An aluminum salt soluble in the concentration range mentioned ispreferably used as the source of aluminum ions. For example, aluminumnitrate and, in particular, aluminum sulfate are suitable for thispurpose, aluminum chloride being less preferred in the interests ofcorrosion control.

Depending on the surface condition of the cans before cleaning, theadditional presence in the cleaning solution of one or more monobasic,dibasic or tribasic hydroxycarboxylic acids containing 4 to 7 carbonatoms in the molecule (the sum total of hydroxyl and carboxyl groupsbeing at least 3) as a further active ingredient or auxiliary in aconcentration of 200 to 800 ppm can have favorable effects. Thehydroxycarboxylic acid(s) is/are preferably selected from monobasic ordibasic hydroxycarboxylic acids containing 6 carbon atoms and at least 4hydroxyl groups. Gluconic acid is particularly preferred. It does notmatter whether the acids are used as such or in the form of their saltssoluble in the concentration range mentioned, more particularly in theform of their sodium salts. At the pH value of the cleaning solution inthe range from 3 to 6, the acids will be present partly in their acidform and partly as carboxylate anions, depending on their acidconstants.

The complex fluorides of the elements boron, titanium, zirconium and/orhafnium may also be used in the form of their acids, for exampletetrafluoroboric acid or the hexafluoro acids of titanium, zirconium andhafnium, or in the form of salts soluble in the concentration rangementioned, for example the alkali metal salts. Since these complexfluorides represent anions of strong acids, they will largely be presentin ionic form in the pH range from 3 to 6.

In a particularly preferred embodiment, the cleaning solution containscomplex fluorides of boron in addition to complex fluorides of at leastone of the metals titanium, zirconium and hafnium, particularlyzirconium. A particularly preferred cleaning solution contains complexfluorides of boron and of zirconium in a ratio by weight of 4:1 to 1:1and, more particularly, in a ratio by weight of 3:1 to 1.5:1.

Preferred nonionic surfactants are surfactants or surfactant mixtureswhich have a cloud point below about 40 to about 45° C. The cleaningsolution may thus be applied by spraying at a working temperature ofabout 50 to about 70° C. without excessive and troublesome foamingoccurring. Suitable surfactants are, in particular, ethoxylates andethoxylates/propoxylates of alkanols containing about 10 to about 18carbon atoms. The ethoxylates and/or the ethoxylates/propoxylates mayalso be end-capped and may be present, for example, as butyl ethers. Theethoxylates preferably contain 4 to 12 ethylene oxide groups and, moreparticularly, about 6 to 10 ethylene oxide groups while theethoxylates/propoxylates preferably contain 3 to 7 ethylene oxide groupsand 2 to 6 propylene oxide groups, preferably 4 to 6 ethylene oxidegroups and 3 to 5 propylene oxide groups. The alkanol component may be apure compound having a certain carbon chain length. However, it iseconomically more attractive to use alkanols of oleochemical origin(oxoalcohols) in which different alkanols with various carbon chainlengths are present. For example, the alkanol component may be a fattyalcohol mixture containing 12 to 14 carbon atoms or an oxoalcoholcontaining 12 to 15 carbon atoms. A particularly preferred surfactantmixture contains both alkanol ethoxylates and alkanolethoxylates/propoxylates, for example in a ratio by weight of 1:3 to1:1.

The corrosion inhibitor(s) may be selected, for example, from mono-, di-or triethanolamine, aromatic carboxylic acids, pyridine or pyrimidinederivatives and diethyl thiourea. Among the ethanolamines,triethanolamine is particularly preferred for toxicological reasons(avoidance of nitrosamine formation). Particularly suitable aromaticcarboxylic acids are benzoic acid and substitution products thereof.Examples include methyl benzoic acids, nitrobenzoic acids, aminobenzoicacids, for example anthranilic acid or p-aminobenzoic acid, andhydroxybenzoic acids, for example salicylic acid. If the treated cansare to be used for foods, pyridine or pyrimidine derivatives and diethylthiourea are less preferred. One example of a suitable inhibitorcombination is a mixture of triethanolamine and benzoic acid, forexample in a ratio by weight of 3:1 to 1:3. However, triethanolamine mayalso be used as sole corrosion inhibitor.

If the complex fluorides are used in the form of their acids in thepreparation of the cleaning solution, it may be necessary to raise thepH value to the required range of about 3 to about 6 and, preferably,about 4 to about 5 by addition of a base. Basic alkali metal compounds,for example hydroxides or carbonates, are suitable for this purpose.However, ammonia is preferably used for adjusting the pH value.

In another embodiment, the present invention relates to the use of thecleaning solution characterized above for cleaning, corrosion-proofingand/or improving lacquer adhesion to articles of tin-plated steel, moreparticularly food or beverage cans. This process has the advantage overthe conventional process that cleaning and temporary corrosion controlcan be achieved in a single treatment stage. The corrosion controlprevents corrosion of the metal surfaces before lacquering, such asmight occur, for example, in the event of plant stoppages. At the sametime, lacquer adhesion and corrosion control after lacquering are bothimproved without any need for a further treatment stage after thecleaning stage. After the treatment with the cleaning solution, the cansare normally rinsed with water, dried at elevated temperature and thenlacquered.

The present invention also relates to a process for cleaning,corrosion-proofing and/or improving lacquer adhesion to articles oftin-coated steel, more particularly food or beverage cans, in which thecans are treated with the cleaning solution described above for a periodof about 30 to about 150 seconds at a temperature in the range fromabout 50 to about 70° C. The treatment may be carried out by sprayingthe cans with the cleaning solution or by dipping the cans in thecleaning solution. Spray cleaning is preferred.

The cleaning solution according to the invention may in principle beprepared by mixing the individual components together in situ in theconcentration ranges mentioned above. In practice, however, suchsolutions are normally marketed in the form of aqueous concentrateswhich may be adjusted to the required concentration range by the user insitu by dilution with water. Accordingly, the present invention alsorelates to a water-based concentrate which, when mixed with water in aconcentration of about 0.5 to about 2.5% by weight, forms the cleaningsolution according to the invention. Besides water or an aqueoussolution of other active ingredients or auxiliaries, this concentratepreferably contains

1 to 4% by weight of complex fluorides of the elements boron, titanium,zirconium and/or hafnium;

1 to 20% by weight of nonionic surfactants;

1 to 10% by weight of corrosion inhibitor.

The concentrate preferably contains as active ingredients: 1.5 to 3% byweight of complex fluorides of the elements boron, titanium, zirconiumand/or hafnium;

0.5 to 3% by weight of aluminum ions; and

3 to 10% by weight of nonionic surfactants;

1.5 to 5% by weight of corrosion inhibitor.

An aluminum-containing concentrate preferably contains 2 to 8% by weightof one or more monobasic, dibasic or tribasic hydroxycarboxylic acidscontaining 4 to 7 carbon atoms in the molecule (the sum of hydroxyl andcarboxyl groups being at least 3) as further active ingredients orauxiliaries.

The foregoing observations apply to the preferred choice of theindividual components. To make the concentrate easier to prepare and toincrease its stability in storage, one or more solubilizers arepreferably present in addition to the actual active ingredients,preferably in a concentration range of about 1 to about 10% by weightand more preferably in a concentration range of about 3 to about 7% byweight. Suitable solubilizers are known substances, for example xylenesulfonates, alkyl phosphates (for example Triton® H66, a product ofUnion Carbide) and, in particular, cumene sulfonate. These anionicsolubilizers are preferably used in the form of alkali metal salts, forexample sodium and/or potassium salts.

EXAMPLES Example 1

A cleaner concentrate according to the invention with the followingcomposition was prepared by mixing the individual components in thefollowing order:

    ______________________________________                                        water                    70.8% by weight                                        fluoroboric acid 1.1% by weight                                               potassium hexafluorozirconate 0.7% by weight                                  aluminum sulfate · 17 H.sub.2 O 12.4% by weight                      sodium gluconate 3.3% by weight                                               C.sub.12/14  fatty alcohol × 5 ethylene oxide × 3.7% by                                  weight                                                 4 propylene oxide                                                             C.sub.12-15  oxoalcohol × 8 ethylene oxide 1.2% by weight                                       Na cumene sulfonate (40% solution) 4.3% by                                   weight                                                 triethanolamine 2.5% by weight                                              ______________________________________                                    

Aqueous cleaning solutions with a pH value of 4 to 4.5 were preparedfrom this concentrate in various concentrations and were used to cleantin cans soiled by residues of corrosion-inhibiting oils anddeep-drawing lubricants by spraying for various periods at a temperatureof 63° C. The cleaning effect was evaluated by visual assessment of thesurface area free from water breaks (0: no cleaning, 100%: goodcleaning). The results are set out in Table 1.

                  TABLE 1                                                         ______________________________________                                        CLEANING EFFECT OF THE CONCENTRATE OF EXAMPLE 1                                 MIXED IN VARIOUS CONCENTRATIONS                                                                           % of Area Free from                               Concentration Spraying Time Water Breaks                                    (% by Weight)                                                                              (seconds)    Outside/Inside                                      ______________________________________                                        0.7          45           60-65/   100                                           90 85-90/ 100                                                                 60 75/ 100                                                                   0.9 45 75/ 85-90                                                               60 85-90/ 100                                                                 90 100/ 100                                                                  1.2 45 90/ 100                                                                 60 100/ 100                                                                ______________________________________                                    

Example 2

To test the corrosion-inhibiting effect, concentrates according toExample 1 were prepared without the corrosion inhibitor,triethanolamine, or with complete or partial replacement thereof.Differences in the composition were made up with water. The concentrateswere diluted with water to give a concentration of 1.2% by weight andthe ready-to-use cleaning solution was sprayed onto tin cans for 60seconds at a temperature of 63° C. The cans were then left standing inthe spray compartment for 10 minutes without rinsing. The formation ofthin-film rust was visually evaluated on a scale of scores: 6=very poor,1=very good. The results are set out in Table 2.

                  TABLE 2                                                         ______________________________________                                        CORROSION-INHIBITING EFFECT                                                       Corrosion Inhibitor in the Concentrate                                      (% by Weight) Evaluation Score                                              ______________________________________                                        None                   6                                                        2.5 Triethanolamine 3                                                         0.83 Triethanolamine + 083 benzoic acid 3.5                                   1.67 Triethanolamine + 1.67 benzoic acid 2                                    1.67 Triethanolamine + 1.67 benzoic acid + 1.5                                0.8 diethyl thiourea                                                          2.5 Benzoic acid 3.5                                                          1.5 Triethanolamine + 1.5 salicylic acid 2                                  ______________________________________                                    

Example 3

To test the long-term corrosion-inhibiting effect after lacquering, tincans were cleaned with various solutions, rinsed for 15 seconds with tapwater and with deionized water, dried in a drying cabinet at 170° C. andlacquered once (two lacquer coatings are normally applied in practice).Two hundred eighty-eight (288) correspondingly treated cans were filledwith Coca Cola® (Coke® and stored for 4 months. The number of rustedcans was then determined. The results are set out in Table 3.

                  TABLE 3                                                         ______________________________________                                        CORROSION TEST (288 CANS FILLED WITH COCA COLA ®)                                 Cleaning*)                                                                              Perforated cans                                             ______________________________________                                        Comp. 1       40                                                                Comp. 2 20                                                                    Example 1 14                                                                ______________________________________                                         *) Comp. 1: tap water only.                                                   Comp. 2: commercial alkaline spray cleaner based on NaOH, gluconate,          nonionic surfactants, corrosion inhibitor. Used as directed.                  Example 1 concentrate of Example 1, 1.2% by weight in water, temperature      63° C., pH 4.3, spraying for 60 seconds.                          

Example 4

A cleaning concentrate according to the invention with the followingcomposition was prepared by mixing the individual components together inthe following order:

    ______________________________________                                        water                    68.1% by weight                                        fluoroboric acid (49% solution) 1.1% by weight                                potassium hexafluorozirconate 0.7% by weight                                  aluminum sulfate · 18 H.sub.2 O 12.4% by weight                      sodium gluconate 3.3% by weight                                               C.sub.12/14  fatty alcohol × 5 ethylene oxide × 3.7% by                                  weight                                                 4 propylene oxide                                                             C.sub.12/14  fatty alcohol × 6 ethylene oxide 1.2% by weight                                    Na cumene sulfonate (40% solution) 5.3% by                                   weight                                                 triethanolamine 4.2% by weight                                              ______________________________________                                    

A 1.1% by weight aqueous solution with a pH value of 4.6 was preparedfrom this concentrate. Tin cans were cleaned with this solution byspraying for 1 minute at 60° C. and were then rinsed for 15 seconds withtap water and with deionized water, dried in a drying cabinet at 170° C.and lacquered twice. A lacquer adhesion test was then carried out bothon the side and on the neck of the cans. To this end, the cans wereplaced in a boiling 1% detergent solution for 30 minutes, rinsed withwater and dried. The lacquer was then cross-hatched and Scotch tape (No.610) was applied and peeled off. Lacquer adhesion was generally completeapart from a few cases where it was substantially complete.

The invention claimed is:
 1. A process for achieving at least one ofcleaning, corrosion-proofing, and improving lacquer adhesion totin-plated steel, said process comprising contacting the tin-platedsteel with an aqueous solution having a pH value from 3 to 6 andcontaining water and:100 to 400 ppm of complex fluorides selected fromthe group consisting of complex fluorides of boron, titanium, zirconium,hafnium, and mixtures of any two or more of boron, titanium, zirconium,and hafnium; 100 to 2,000 ppm of nonionic surfactants; and 100 to 1,000ppm of corrosion inhibitor; and, optionally, one or more of thefollowing components: aluminum ions, hydroxycarboxylic acids andsolubilizers.
 2. A process as claimed in claim 1, wherein said aqueoussolution contains:150 to 300 ppm of complex fluorides of boron,titanium, zirconium, hafnium, and mixtures of any two or more of boron,titanium, zirconium, and hafnium; 50 to 300 ppm of aluminum ions; 300 to1,000 ppm of nonionic surfactants; and 150 to 500 ppm of corrosioninhibitor.
 3. A process as claimed in claim 2, wherein said aqueoussolution also contains:200 to 800 ppm of one or more monobasic, dibasicor tribasic hydroxycarboxylic acids having from 4 to 7 carbon atoms ineach molecule and a sum of hydroxyl and carboxyl groups in each moleculethat is at least
 3. 4. A process as claimed in claim 3, wherein thehydroxycarboxylic acid molecules are selected from monobasic or dibasichydroxycarboxylic acid molecules each containing 6 carbon atoms and atleast 4 hydroxyl groups.
 5. A process as claimed in claim 4, whereinsaid aqueous solution contains complex fluorides of both boron andzirconium in a ratio by weight of 4:1 to 1:1.
 6. A process as claimed inclaim 5, wherein the nonionic surfactants are selected from the groupconsisting of (i) alkanol ethoxylates having from 10 to 18 carbon atomsand 4 to 12 oxyethylene moieties in each molecule and (ii) alkanolethoxylate/propoxyplates having from 10 to 18 carbon atoms, 3 to 7oxyethylene moieties and 2 to 6 oxypropylene moieties in each molecule.7. A process as claimed in claim 6, wherein the corrosion inhibitors areselected from mono-, di- or tri-ethanolamine, aromatic carboxylic acids,pyridine or pyrimidine derivatives and diethyl thiourea.
 8. A process asclaimed in claim 7, wherein the tin-plate surfaces contacted are thoseof food or beverage cans.
 9. A process as claimed in claim 1, whereinsaid aqueous solution also contains:200 to 800 ppm of one or moremonobasic, dibasic or tribasic hydroxycarboxylic acids having from 4 to7 carbon atoms in each molecule and a sum of hydroxyl and carboxylgroups in each molecule that is at least
 3. 10. A process as claimed inclaim 9, wherein the hydroxycarboxylic acid molecules are selected frommonobasic or dibasic hydroxycarboxylic acid molecules each containing 6carbon atoms and at least 4 hydroxyl groups.
 11. A process as claimedclaim 1, wherein said aqueous solution contains complex fluorides ofboth boron and zirconium in a ratio by weight of 4:1 to 1:1.
 12. Aprocess as claimed in claim 1, wherein the nonionic surfactants areselected from the group consisting of (i) alkanol ethoxylates havingfrom 10 to 18 carbon atoms and from 4 to 12 oxyethylene moieties in eachmolecule and (ii) alkanol ethoxylate/propoxylates having from 10 to 18carbon atoms, 3 to 7 oxyethylene moieties and 2 to 6 oxypropylenemoieties in each molecule.
 13. A process as claimed in claim 1, whereinthe corrosion inhibitors are selected from mono-, di- ortri-ethanolamine, aromatic carboxylic acids, pyridine or pyrimidinederivatives and diethyl thiourea.
 14. A water-based concentrate forpreparing a cleaning solution by dilution with water, said concentratecomprising water and:1.5 to 3% by weight of one or more substancesselected from the group consisting of complex fluorides of boron,titanium, zirconium, hafnium, and mixtures of any two or more of boron,titanium, zirconium, and hafnium; 0.5 to 2% by weight of aluminum ions;3 to 10% by weight of nonionic surfactants; and 1.5 to 5% by weight ofcorrosion inhibitor.
 15. A water-based concentrate as claimed in claim14, which additionally contains 1 to 10% by weight of a solubilizer. 16.A water-based concentrate as claimed in claim 14, which also contains:2to 8% by weight of one or more monobasic, dibasic or tribasichydroxycarboxylic acids having from 4 to 7 carbon atoms in each moleculeand a sum of hydroxyl and carboxyl groups in each molecule that is atleast
 3. 17. A water-based concentrate as claimed in claim 16, whichadditionally contains:1 to 10% by weight of a solubilizer.
 18. Awater-based concentrate as claimed in claim 16, wherein thehydroxycarboxylic acid molecules are selected from monobasic or dibasichydroxycarboxylic acid molecules each containing 6 carbon atoms and atleast 4 hydroxyl groups.
 19. A process according to claim 8 whereincontacting the cans is for 30 to 150 seconds at a temperature of 50 to70° C. by spraying the cans with the cleaning solution or by dipping thecans in the cleaning solution.
 20. An aqueous cleaning solution fortin-plated steel which has a pH value from 3 to 6 and contains waterand:150 to 300 ppm of complex fluorides selected from the groupconsisting of complex fluorides of boron, titanium, zirconium, hafnium,and mixtures of any two or more of boron, titanium, zirconium, andhafnium; 50 to 300 ppm of aluminum ions; 300 to 1,000 ppm of nonionicsurfactants; and 50 to 500 ppm of corrosion inhibitor; and, optionally,one or more of hydroxycarboxylic acids and solubilizers.